1. Field of the Invention
The present invention relates to a cyclometalated transition metal complex and an organic electroluminescent display device using the same, and more particularly, to a cyclometalated transition metal complex that can emit light at a wavelength range from the blue region to the red region from triplet metal-to-ligand charge-transfer (3MLCT) state, and an organic electroluminescent display device that applies the complex as an organic film forming material.
2. Description of the Related Art
An organic electroluminescent display device (organic electroluminescence (EL) display device) is an active light-emitting display device employing the phenomenon that when an electric current is applied to a thin film (hereinafter referred to as “organic film”) composed of a fluorescent or phosphorescent organic compound, the fluorescent or the phosphorescent organic compound emits light in response to the recombination of electrons and holes in the organic film. The display device is light, has a structure of which the components are simple and its manufacturing process is simple, and ensures wide view angle with high image quality. Further, the display has the electrical properties suitable for portable electronic devices, since it can embody completely high color purity and moving picture, and can be driven with low power consumption and low voltage.
A general organic electroluminescent display device has a structure that has an anode formed at the upper part of a substrate, and a hole transporting layer, a light emitting layer, an electron transporting layer and a cathode sequentially formed at the upper part of the anode. Herein, the hole-transporting layer, the light emitting layer and the electron-transporting layer are organic films composed of organic compounds. The driving principle for the organic electroluminescent display device having such a structure is as follows. When voltage is applied between the anode and the cathode, a hole injected from an anode is migrated to a light-emitting layer via a hole-transporting layer. Meanwhile, an electron is injected from a cathode into a light-emitting layer via an electron-transporting layer, and carriers are recombined at the area of the light-emitting layer to form an exciton. The exciton emits light with a wavelength corresponding to a band gap of the material when the exciton decays radiatively.
The light emitting layer-forming materials are classified into a fluorescent material using singlet excitons and a phosphorescent material using triplet excitons, according to their light-emitting mechanism. The light emitting layer is formed of the fluorescent or phosphorescent material alone or an appropriate host material doped with the fluorescent or phosphorescent material, and as electrons are excited, singlet excitons and triplet excitons are formed in the host. Herein, the statistic-forming ratio of the singlet excitons to the triplet excitons is 1:3.
The organic electroluminescent display device using a fluorescent material as a light emitting layer-forming material has a disadvantage that the triplet excitons formed in the host are wasted, while the device using a phosphorescent material as a light emitting layer-forming material has an advantage that both of the singlet excitons and the triplet excitons can be used, and thus the internal quantum efficiency can reach 100% (Baldo, et al., Nature, Vol. 395, 151-154, 1998). Accordingly, the phosphorescent material can possess even higher light emitting efficiency when a phosphorescent material is used as a light emitting layer-forming material than when a fluorescent material is used.
When a heavy metal such as Ir, Pt, Rh, Pd, etc. is incorporated into an organic molecule, triplet state and singlet state are mixed through spin-orbital coupling occurred by the heavy metal atom effect. Due to this, the transition that had been blocked is possible, and the phosphorescence can be occurred efficiently even at room temperature.
Recently, a green material or a red material with high efficiency employing the phosphorescence of which the internal quantum efficiency reaches 100% was developed.
Although transition metal compounds comprising transition metals such as an iridium, a platinum, etc. as a highly efficient luminescent material employing phosphorescence are reported, the materials that satisfy the properties required for realizing a full color display with high efficiency or white light emitting with low power consumption are limited to the green and red regions, and a phosphorescent material suitable for the blue region is not developed. For such reason, there is an obstacle in developing a phosphorescent full color device.
To solve such problems, a blue light emitting material is developing (WO02/15645 A1, U.S. Patent Publication No. 2002/0064681 A1). Further, an organic metal complex, incorporating a bulky functional group that can make the HOMO-LUMO difference large by changing the molecular geometry, or a functional group that has a strong ligand field (e.g., cyano group), was developed. Besides, an iridium complex represented by formula Ir(ppy)2P(ph)3Y (wherein Y=Cl or CN) (U.S. Patent Publication No. 2002/0182441 A1), and an iridium (III) complex having a cyclometalated ligand, a chelating diphosphine, a chlorine and a cyano group (U.S. Patent Publication No. 2002/0048689 A1) were developed.
Further, the US Patent Publication No. 2002/0134984 discloses a cyclometalated transition metal complex comprised of nitrogen atoms and carbon atoms, and an organic electroluminescent display device comprising the same.
However, all of the above materials do not show satisfactory properties in terms of color purity, light emitting efficiency and lifetime, etc.